S. Perna

Large Hysteresis effect in Synchronization of Nanocontact Vortex Oscillators by Microwave Fields.

Current-induced vortex oscillations in an extended thin-film with point-contact geometry are considered. The synchronization of these oscillations with a microwave external magnetic field is investigated by a reduced order model that takes into account the dynamical effects associated with the significant deformation of the vortex structure produced by the current, which cannot be taken care of by using the standard rigid vortex theory. The complete phase diagram of the vortex oscillation dynamics is derived and it is shown that strong hysteretic behavior occurs in the synchronization with the external field. The complex nonlinear nature of the synchronization manifests itself also through the appearance of asymmetry in the locking frequency bands for moderate microwave field amplitudes. Predictions from the reduced order model are confirmed by full micromagnetic simulations.

Analysis of reliable sub-ns spin-torque switching under transverse bias magnetic fields

The switching process of a magnetic spin-valve nanosystem subject to spin-polarized current pulses is considered. The dependence of the switching probability on the current pulse duration is investigated. The further application of a transverse field along the intermediate anisotropy axis of the particle is used to control the quasi-random relaxation of magnetization to the reversed magnetization state. The critical current amplitudes to realize the switching are determined by studying the phase portrait of the Landau-Lifshtz-Slonczewski dynamics. Macrospin numerical simulations are in good agreement with the theoretical prediction and demonstrate reliable switching even for very short (below 100 ps) current pulses.

Phase-Flow Interpretation of Magnetization Relaxation in Nanomagnets

This paper is devoted to the study of the probability of relaxation of a bistable nanomagnet from high energy to one of its two energy minima. The evolution in phase space of a Gibbs ensemble of replicas of the nanomagnet is analyzed in the limit of small Gilbert damping and in the absence of thermal noise. It is shown that when an external magnetic field is applied along the intermediate anisotropy axis, the relaxation probability exhibits a saw-tooth dependence on the field amplitude, periodically reaching the extreme values 0 or 1, in correspondence to which relaxation is fully insensitive to randomness in initial conditions.

Influence of the Second-Order Uniaxial Anisotropy on the Dynamical Proprieties of Magnetic Tunnel Junctions

The magnetization dynamics of a ferromagnet can be influenced by the second-order anisotropy contribution when the magnetocrystalline perpendicular anisotropy is of the same order of the out-of-plane demagnetizing field (compensation point). This condition can be fulfilled in magnetic tunnel junctions (MTJs) where the induced interfacial perpendicular anisotropy can be controlled by the thickness of the free layer. This paper describes, within a both micromagnetic and analytical/numerical framework, how switching and self-oscillation of the magnetization are affected by the second order magnetocrystalline anisotropy near the compensation point. We have derived analytical expressions for the switching critical current and the oscillation frequencies that are well reproduced by numerical simulations. Our results can support the design of MTJs for application either in storage and nanoscale signal generators.

Current-Driven Hysteretic Synchronization in Vortex Nanopillar Spin-Transfer Oscillators

The synchronization of nanopillar vortex spin-transfer nano-oscillators with external ac injected current is considered. A collective variables description is used to derive an analytical reduced-order model for the vortex core dynamics. This model is able to predict all the possible vortex oscillation regimes as well as the transition mechanisms among them. In particular, it is shown that hysteretic synchronization occurs at moderate ac currents. The analytical results are in good agreement with micromagnetic simulations.

Noise-induced bifurcations in magnetization dynamics of uniaxial nanomagnets

Stochastic magnetization dynamics in uniformly magnetized nanomagnets is considered. The system is assumed to have rotational symmetry as the anisotropy axis, the applied field, and the spin polarization are all aligned along an axis of symmetry. By appropriate integration of the Fokker-Planck equation associated to the problem, the stochastic differential equation governing the evolution of the angle between the magnetization orientation and the symmetry axis is derived. The drift terms present in this equation contain a noise-induced drift term, which, in combination with drift terms of deterministic origin, can be written as the derivative of an effective potential. Superparamagnetic-like transitions are studied in connections with the bifurcations of the effective potential as temperature and excitation conditions are varied.

Heteroclinic tangle phenomena in nanomagnets subject to time-harmonic excitations

Magnetization dynamics in uniformly magnetized nanomagnets excited by time-harmonic (AC) external fields or spin-polarized injected currents is considered. The analysis is focused on the behaviour of the AC-excited dynamics near saddle equilibria. It turns out that this dynamics has a chaotic character at moderately low power level. This chaotic and fractal nature is due to the phenomenon of heteroclinic tangle which is produced by the combined effect of AC-excitations and saddle type dynamics. By using the perturbation technique based on Melnikov function, analytical formulas for the threshold AC excitation amplitudes necessary to create the heteroclinic tangle are derived. Both the cases of AC applied fields and AC spin-polarized injected currents are treated. Then, by means of numerical simulations, we show how heteroclinic tangle is accompanied by the erosion of the safe basin around the stable regimes.

Effect of Temperature in Hysteretic Synchronization of Magnetic Vortex Spin-Torque Nano-Oscillators

The synchronization of vortex spin-transfer nano-oscillators (both nanopillar and nanocontact structures) with external microwave excitations is considered. A collective variables description is used to derive an analytical reduced-order model for the vortex core dynamics, which is able to predict all the possible oscillation regimes and hysteretic transitions among them. The influence of thermal fluctuations on hysteretic synchronization is studied by full micromagnetic simulations. The numerical results are in good agreement with the theory.

Analytical Treatment of Nonlinear Ferromagnetic Resonance in Nanomagnets

We study magnetization oscillations in a uniformly magnetized nanomagnet driven by ac excitations (external fields and injected currents) when the frequency of excitation is close to ferromagnetic resonance frequency. By using separation of time scale and the averaging technique, we derive dynamical equations, which can be studied by the methods of dynamical systems theory. This leads to an analytical description of nonlinear frequency response and to the analysis of relevant bifurcation phenomena. The theoretical results are compared with numerical simulations.

Analysis of Reliable Ultrafast Precessional Switching in the Presence of Transverse Applied Magnetic Fields

The precessional switching process of a magnetic nanoparticle subject to external field pulses applied along the hard-axis is considered. The dependence of the switching probability on the external field pulse duration is investigated. The further application of a transverse field along the intermediate anisotropy axis of the particle is used to control the quasi-random relaxation of magnetization to the reversed magnetization state. The critical field amplitudes to realize the switching are analytically determined. The robustness of the theoretical prediction is verified by macrospin numerical simulations.